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Valid Saturday September 22, 2018 to Wednesday October 03, 2018
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EDT September 19 2018Synopsis
: Several upper-level low pressure
systems are forecast to move over the northern tier of the U.S. during the next
two weeks, causing possible heavy rain and some convective weather across parts
of the lower 48 states. Much of the central U.S. is likely to be wet through
the 3-7 day period, with possible extension into Week-2. Widespread upper-level
high pressure in Alaska is likely to be disrupted by a shortwave trough moving
over the interior at the beginning of the 3 to 7 day period, while surface low
pressure over the Bering Sea, Aleutians, and South Coast of Alaska is expected
to cause onshore flow along the Gulf of Alaska for Week-2.
Detailed Summary For Saturday September 22 - Wednesday September 26:
- Heavy rain across portions of the Middle
Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains,
Sat-Sun, Sep 22-Sep 23.
- Heavy rain across portions of the Southern Appalachians, the Central
Appalachians, the Ohio Valley, and the Tennessee Valley, Mon-Tue, Sep 24-Sep 25.
- Heavy rain across coastal portions of the South Coast of Alaska, the Kenai
Peninsula, and northern Alaska Panhandle, Tue-Wed, Sep 25-Sep 26.
- Flooding possible across portions of the Great Lakes, the Middle
Mississippi Valley, the Mid-Atlantic, the Central Appalachians, and the Upper
- Flooding occurring or imminent across portions of the Southeast, the
Central Appalachians, the Great Lakes, the Mid-Atlantic, and the Southern
- Flooding likely across portions of the Southeast and the Mid-Atlantic.
- Severe Drought across the Central Plains, the Central Rockies, the Lower
Mississippi Valley, the Central Great Basin, the Northern Plains, the Southern
Plains, Hawaii, the Northern Great Basin, the Southern Rockies, the Middle
Mississippi Valley, the Northeast, California, the Northern Rockies, the Upper
Mississippi Valley, the Pacific Northwest, and the Southwest.
- Slight risk of much below normal temperatures for portions of the Central
Plains, the Northern Plains, the Northern Rockies, and the Upper Mississippi
Valley, Thu-Mon, Sep 27-Oct 1.
- Slight risk of heavy precipitation for portions of the Lower Mississippi
Valley, the Tennessee Valley, the Middle Mississippi Valley, and the Southern
Plains, Thu-Sat, Sep 27-Sep 29.
- Moderate risk of heavy precipitation for portions of the Southern Plains
and Lower Mississippi Valley, Thu-Sat, Sep 27-29.
- Slight risk of heavy precipitation for coastal portions of the South Coast
of Alaska, Thu-Mon, Sep 27-Oct 1.
Surface troughing is anticipated across parts of Texas, as well as a stationary
front extending from the Southern Plains northeastward to the Mid-Atlantic
throughout the beginning to middle of the 3 to 7 day period. This pattern is
expected to support heavy rains across parts of the Middle Mississippi Valley,
the Lower Mississippi Valley, and the Southern Plains, Sep 22 to 23. As it
shifts further east, this stationary front, in addition to a cold front forming
across the Southern Plains and Upper Mississippi Valley is likely to bring
heavy rainfall to parts of the Southern Appalachians, the Central Appalachians,
the Ohio Valley, and the Tennessee Valley, Sep 24-25. Highlighted areas may
receive up to or greater than 1.5 inches of rainfall in 24-hours. A surface low
is predicted to form along the northern portion of this cold front and bring
heavy rainfall to localized parts of the Upper Mississippi Valley. However, due
to significant model differences regarding rainfall amounts and spatial
disagreement regarding heaviest rainfall, an additional heavy rain area is not
identified at this time.
Flooding in the Mid-Atlantic and Southeast is likely to continue through
the end of this weekend, as the region recovers from the impacts of Hurricane
Florence and its remnant low. This has led to record flooding for large
portions of the Carolinas.
Surface low pressure is predicted to form along the Aleutians by Sep 25,
favoring strong onshore flow from the Gulf of Alaska into the South Coast of
Alaska. This pattern favors heavy rainfall across coastal portions of the South
Coast of Alaska, the Kenai Peninsula, and northern Alaska Panhandle, Sep 25 to
26. Some models are indicating the potential for localized areas receiving up
to or greater than 3 inches of rainfall in a 24-hour period. This is a
significant pattern change compared to the ongoing anomalously dry conditions
in this area. For Thursday September 27 -
Wednesday October 03:
An amplified upper-level trough is expected to
dominate the eastern two-thirds of the CONUS throughout much of Week-2. This
trough may support frontal development across the central Great Plains at the
beginning of Week-2. A slight risk of heavy precipitation has been posted over
parts of the Lower Mississippi Valley, the Middle Mississippi Valley, and the
Southern Plains for the early portion of Week-2, Sep 27 to 29. A moderate risk
area for heavy precipitation is highlighted across parts of the Southern Plains
and Lower Mississippi Valley, Sep 27 to 29, where the greatest chance for heavy
precipitation is forecast. The identified areas indicate where the GEFS and
ECMWF Reforecast Tools indicate a 20% or greater chance of 3-day precipitation
accumulations reaching the upper 85th percentile, compared to climatology.
The amplified trough is also likely to cause some of the colder air from
Canada to spill south over the Great Plains and neighboring regions. A slight
risk for much below normal temperatures has been posted for portions of the
Central Plains, the Northern Plains, the Northern Rockies, and the Upper
Mississippi Valley Sep 27 to Oct 1. First freezes are possible for portions of
the northern Plains.
Antecedent amplified ridging over Alaska is anticipated to continue into
the beginning and middle of Week-2, weakening towards the end of Week-2, with
the possibility of some upper-level low pressure moving in. The GFS and ECMWF
probabilistic extremes tool show good agreement regarding an increased
likelihood for heavy precipitation along parts of coastal portions of the South
Coast of Alaska throughout Week-2, where a slight risk for heavy precipitation
has been forecast. Model guidance shows several surface low pressure systems
moving through the Gulf of Alaska during Week-2, supporting onshore flow and
leading to prolonged periods of heavy rain. For most of the Interior, warm
temperatures are forecast throughout Week-2, with the GEFS probabilities tool
indicating a 60% a widespread chance of exceeding the 90th climatological
percentile toward the end of the week. Although these temperatures are not
inherently hazardous, associated coastal flooding and other risks should be
The U.S. Drought Monitor, valid on September 11, indicates a continued
decrease in severe to exceptional drought (D2 to D4) coverage from 18.86
percent last week to 16.7 percent this week. Recent heavy rainfall resulted in
improvement across the middle Mississippi Valley along with central and
southern Great Plains.
Forecaster: Melissa Ou
Click here to see a display of the GFS Ensemble Forecasts
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.